The present invention relates to a novel mutant allele of Beta vulgaris designated ACSDMR001, which results in a high resistance to the downy mildew pathogen Peronospora farinose f. sp. betae (syn. P. schactii). The present invention also relates to B. vulgaris seed, B. vulgaris plants and parts thereof, and B. vulgaris varieties and hybrids which contain the mutant allele. In addition, the present invention is directed to transferring the ACSDMR001 mutant allele to other plants in the same genus lacking the allele, and is useful for producing novel types and varieties of downy mildew resistant Beta vulgaris. All publications cited in this application are herein incorporated by reference.
The beet (Beta vulgaris) is a plant in the Amaranthaceae family. Beet is best known for its numerous cultivated varieties, the most common one being the purple root vegetable known as beetroot or garden beet. However, other cultivated varieties in the Beta genus include the leaf vegetables chard and spinach beet, as well as the root vegetables sugar beet, which is important in the production of table sugar, and magelwurzel, which is a fodder crop. Three subspecies are typically recognized: Beta vulgaris subsp. vulgaris, Beta vulgaris subsp. maritima, and Beta vulgaris subsp. adanensis. All cultivated varieties fall into the subspecies Beta vulgaris subsp. vulgaris, while Beta vulgaris subsp. maritima, commonly known as the sea beet, is the wild ancestor of these and is found throughout the Mediterranean, the Atlantic coast of Europe, the Near East, and India. Another wild subspecies, Beta vulgaris subsp. adanensis, occurs from Greece to Syria.
Beta vulgaris is an herbaceous biennial or, rarely, perennial plant with leafy stems. Garden beets are grown for the roots, which are eaten cooked, as a vegetable, in salads or pickled. Chard and spinach beet are grown for the leaves, which are used as a potherb and in salads. The roots and leaves of the beet have also been used in folk medicine to treat a wide variety of ailments.
Downy mildew is a harmful disease of beet that can lead to economic losses in both seed and commercial production fields. Downy mildew is caused by the plant pathogen Peronospora farinosa f. sp. beticola (Pfb), which persists as oospores in the soil, on beet seed crops, or on overwintered volunteer beet plants. Attacks are most damaging at the seedling stage. The cotyledons are systemically infected, becoming discolored and distorted, and loss of seedlings causes uneven crop development. The infection is favored by wet and cool weather conditions, and may appear in early spring and recur in autumn. Under cool wet conditions, sporangia of Pfb can germinate on the leaf surface in 2-6 hours. Infection of leaf tissue is usually completed after 3-4 hours. The development of lesions is favored by temperatures of approximately 20° C. and under these ideal conditions sporulation usually occurs 6-7 days after infection but it can be up to 12 days. Control relies on adequate crop rotations, and avoidance of sources of infection. Beet varieties that exhibit a high level of resistance to downy mildew infection are highly desirable.
There are numerous steps in the development of any novel, desirable plant germplasm. Plant breeding begins with the analysis and definition of problems and weaknesses of the current germplasm, the establishment of program goals, and the definition of specific breeding objectives. The next step is selection of germplasm that possess the traits to meet the program goals. The goal is to combine in a single variety or hybrid an improved combination of desirable traits from the parental germplasm. These important traits may include increased fruit size and weight, higher seed yield, improved color, resistance to diseases and insects, tolerance to drought and heat, and better agronomic quality.
Choice of breeding or selection methods depends on the mode of plant reproduction, the heritability of the trait(s) being improved, and the type of cultivar used commercially (e.g., F1 hybrid cultivar, pureline cultivar, etc.). For highly heritable traits, a choice of superior individual plants evaluated at a single location will be effective, whereas for traits with low heritability, selection should be based on mean values obtained from replicated evaluations of families of related plants. Popular selection methods commonly include pedigree selection, modified pedigree selection, mass selection, and recurrent selection.
The complexity of inheritance influences choice of the breeding method. Backcross breeding is used to transfer one or a few favorable genes for a highly heritable trait into a desirable cultivar. This approach has been used extensively for breeding disease-resistant cultivars. Various recurrent selection techniques are used to improve quantitatively inherited traits controlled by numerous genes. The use of recurrent selection in self-pollinating crops depends on the ease of pollination, the frequency of successful hybrids from each pollination, and the number of hybrid offspring from each successful cross.
Each breeding program should include a periodic, objective evaluation of the efficiency of the breeding procedure. Evaluation criteria vary depending on the goal and objectives, but should include gain from selection per year based on comparisons to an appropriate standard, the overall value of the advanced breeding lines, and the number of successful cultivars produced per unit of input (e.g., per year, per dollar expended, etc.).
Promising advanced breeding lines are thoroughly tested and compared to appropriate standards in environments representative of the commercial target area(s) for at least three years. The best lines are candidates for new commercial cultivars. Those still deficient in a few traits are used as parents to produce new populations for further selection.
These processes, which lead to the final step of marketing and distribution, usually take from many years from the time the first cross or selection is made. Therefore, development of new cultivars is a time-consuming process that requires precise forward planning, efficient use of resources, and a minimum of changes in direction.
A most difficult task is the identification of individuals that are genetically superior, because for most traits the true genotypic value is masked by other confounding plant traits or environmental factors. One method of identifying a superior plant is to observe its performance relative to other experimental plants and to a widely grown standard cultivar. If a single observation is inconclusive, replicated observations provide a better estimate of its genetic worth.
The goal of beet plant breeding is to develop new, unique, and superior beet cultivars. The breeder initially selects and crosses two or more parental lines, followed by repeated selfing and selection, producing many new genetic combinations. The breeder can theoretically generate billions of different genetic combinations via crossing, selfing, and mutations. The breeder has no direct control at the cellular level. Therefore, two breeders will never develop the same line, or even very similar lines, having the same beet traits.
Each year, the plant breeder selects the germplasm to advance to the next generation. This germplasm is grown under different geographical, climatic, and soil conditions, and further selections are then made during, and at the end of, the growing season. The cultivars that are developed are unpredictable. This unpredictability is because the breeder's selection occurs in unique environments, with no control at the DNA level (using conventional breeding procedures), and with millions of different possible genetic combinations being generated. A breeder of ordinary skill in the art cannot predict the final resulting lines he develops, except possibly in a very gross and general fashion. The same breeder cannot produce the same line twice by using the exact same original parents and the same selection techniques. This unpredictability results in the expenditure of large research monies to develop superior beet cultivars.
Descriptions of breeding methods that are commonly used for different traits and crops can be found in one of several reference books (e.g., Principles of Plant Breeding, John Wiley and Son, pp. 115-161 (1960); Allard (1960); Simmonds (1979); Sneep, et al. (1979); Fehr (1987); “Carrots and Related Vegetable Umbelliferae,” Rubatzky, V. E., et al. (1999).
Proper testing should detect any major faults and establish the level of superiority or improvement over current cultivars. In addition to showing superior performance, there must be a demand for a new cultivar that is compatible with industry standards or which creates a new market. The introduction of a new cultivar will incur additional costs to the seed producer, the grower, processor and consumer for special advertising and marketing, altered seed and commercial production practices, and new product utilization. The testing preceding release of a new cultivar should take into consideration research and development costs, as well as technical superiority of the final cultivar. For seed-propagated cultivars, it must be feasible to produce seed easily and economically.
Beet is an important and valuable vegetable crop. Thus, a continuing goal of beet plant breeders is to develop stable, high yielding beet cultivars that are agronomically sound. To accomplish this goal, the beet breeder must select and develop beet plants with traits that result in superior cultivars. The development of beet varieties having an increased level of resistance to downy mildew is very important for improving beet production.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification.